Process and catalyst for production of formaldehyde from dimethyl ether

ABSTRACT

Dimethyl ether is converted to formaldehyde using a supported catalyst comprising molybdenum and/or vanadium oxides. The surface density of the oxide(s) ranges from greater than that for the isolated monomeric oxides upwards, so long as there is a substantial absence of bulk crystalline molybdenum and/or vanadium oxide(s). Conversion and selectivity to formaldehyde are improved as compared to data reported for known catalysts. Also disclosed is a supported catalyst comprising molybdenum and/or vanadium oxides wherein the support comprises one or more reducible metal oxides, preferably a layer or layers of one or more reducible metal oxides disposed on the surface of a particulate alumina or zirconia support.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation-in-part of application Ser. No. 10/081,047filed Feb. 20, 2002.

FIELD AND BACKGROUND OF THE INVENTION

[0002] This invention relates to a process for production offormaldehyde, and optionally also methyl formate as a co-product, byoxidation of dimethyl ether (DME), and to catalysts for use in theprocess, including catalysts that are novel per se. In addition, thisinvention relates to the use of such novel catalysts in other processes.

[0003] Formaldehyde is widely used as an intermediate or basic buildingblock in the commercial synthesis of many chemicals. Because of theexistence of large reserves of methane worldwide it has been considereddesirable for some time to develop processes to convert methane to morevaluable chemicals. One such effort has been in the area of directconversion of methane to formaldehyde via selective oxidation. However,this has not been particularly successful. Up to now, all such processeshave resulted in low yields due to the tendency of the formaldehyde soproduced being further oxidized to carbon oxides under the severereaction conditions required for methane oxidation.

[0004] Instead, formaldehyde is commercially produced from methaneindirectly, for instance, by first converting the methane to synthesisgas (CO and H₂), then reacting that to form methanol, and finallyoxidizing the methanol to produce formaldehyde. The oxidation ofmethanol to formaldehyde has been extensively studied, and is thedominant process today for formaldehyde synthesis, typically usingsilver- or iron/molybdenum-based catalysts.

[0005] Another possible route to formaldehyde involves the oxidation ofdimethyl ether (CH₃OCH₃) via cleavage of the C—O—C linkages. Thisprocess, however, has not been widely studied.

[0006] Dimethyl ether is a generally environmentally benign molecule.Its physical properties resemble those of LPG (liquefied petroleumgases), and dimethyl ether thus can be transported within existing anddeveloping LPG infrastructures. Like methanol, dimethyl ether can beproduced from synthesis gas. These characteristics give it the potentialto be a new, clean alternative fuel. This potential is expected to leadto the production of substantially larger quantities of dimethyl etherthan in the past, thus making it available for use as an intermediate inproduction of other chemicals, including formaldehyde.

[0007] Several patents disclose processes for producing formaldehydefrom dimethyl ether using various catalysts. U.S. Pat. No. 2,075,100describes such a process using a number of comparatively mild oxidationcatalysts including platinum wire or foil, palladium black, and metalssuch as gold, silver, and copper. Vanadium pentoxide and iron, chromiumand uranium sesqui-oxides are termed “very suitable”. U.S. Pat. No.3,655,771 describes using catalysts containing tungsten oxide, alone oroptionally with no more than 10% of an additive. The additives mentionedinclude bismuth, selenium, molybdenum, vanadium, phosphorus and boronoxides, as well as phosphoric acid, ammonium phosphate and ammoniumchloride.

[0008] More recently, U.S. Pat. No. 4,435,602 describes a process forproduction of formaldehyde from dimethyl ether using naturally occurringmanganese nodules as a catalyst. U.S. Pat. No. 4,439,624 describes sucha process using an intimate mixture of bismuth, molybdenum and copperoxides, preferably prepared by coprecipitation. U.S. Pat. No. 4,442,307describes such a process using an intimate mixture of bismuth,molybdenum and iron oxides, similarly prepared. U.S. Pat. No. 6,256,528describes oxidation of dimethyl ether with a catalyst containingmetallic silver to produce a mixture of products including formaldehyde,light alkanes, carbon oxides and water. Information in these patentsindicates that formaldehyde was produced with reasonable yields, butthat overoxidation of that product to carbon oxides occurred to anundesirable degree.

[0009] As described above, it would be advantageous to provide a processand associated process technology for production of formaldehyde fromdimethyl ether with good conversion and good selectivity toformaldehyde. Preferably such a process could be operated without theoccurrence of substantial direct oxidation of dimethyl ether to carbonoxides or further oxidation of product formaldehyde to carbon oxides,thus improving the chemical and energy efficiency of the process.

BRIEF SUMMARY OF THE INVENTION

[0010] In brief, in one aspect, this invention comprises a process forthe production of formaldehyde by oxidation of dimethyl ether in thepresence of a supported catalyst comprising molybdenum oxide, vanadiumoxide or a mixture of molybdenum and vanadium oxides. The support is onethat substantially does not react with the molybdenum or vanadium oxideto form unreducible mixed oxide(s). Preferred supports comprise alumina,zirconia, stannic oxide, titania, silica, ferric oxide, ceric oxide,other reducible metal oxides, and mixtures and combinations thereof.

[0011] In one preferred embodiment this invention comprises such aprocess in which the molybdenum and/or vanadium oxides are dispersed onthe surface of the support, the surface density of the oxide or oxideson the support is greater than that for the isolated monomeric oxide oroxides, and in which the catalyst is characterized by a substantialabsence of bulk crystalline molybdenum and/or vanadium oxides.

[0012] Most preferably the surface density of the molybdenum and/orvanadium oxide or oxides on the support is approximately that of amonolayer of the oxide or oxides at the surface of the support.

[0013] In another preferred embodiment, the catalyst comprises one ormore reducible metal oxides. More preferably in this embodiment, thecatalyst comprises a layer of the reducible metal oxide or oxides, mostpreferably stannic oxide, on a particulate support (preferably aluminaand/or zirconia) with the molybdenum and/or vanadium oxide or oxidesbeing present as an upper layer or layers on the layer of reduciblemetal oxide(s) layer. In this embodiment, preferably the surface densityof the molybdenum and/or vanadium oxide or oxides on the support isgreater than that for the isolated monomeric oxide or oxides, and thecatalyst is characterized by a substantial absence of bulk crystallinemolybdenum and/or vanadium oxides. Most preferably the surface densityof the molybdenum and/or vanadium oxide or oxides on the support isapproximately that of a monolayer of the oxide or oxides at the surfaceof the support.

[0014] Catalysts of the above type in which the catalyst comprises oneor more reducible metal oxides, particularly stannic oxide, and moreparticularly in which the molybdenum and/or vanadium oxide is supportedon a layer or layers of reducible metal oxide or oxides, with the oxidelayer or layers being disposed on a particulate alumina and/or zirconia,are novel and form another feature of this invention.

[0015] Yet another aspect of this invention is the use of the novelcatalysts just described to catalyze other processes, particularlyoxidation of methanol to formaldehyde, oxidative dehydrogenation ofalkanes, and oxidation of alkenes.

DETAILED DESCRIPTION OF THE INVENTION

[0016] In brief, a primary aspect of this invention comprises a processfor the production of formaldehyde by oxidation of dimethyl ether in thepresence of a supported catalyst comprising molybdenum oxide, vanadiumoxide or a mixture of molybdenum and vanadium oxides. Preferably theoxides are supported on alumina (Al₂O₃) and/or zirconia (ZrO₂), and morepreferably on such a support that also includes one or more reduciblemetal oxides, as described herein. Preferably, the molybdenum and/orvanadium oxides are dispersed on the surface of the support, the surfacedensity of the oxide or oxides on the support is greater than that forthe isolated monomeric oxide or oxides, and the catalyst ischaracterized by a substantial absence of bulk crystalline molybdenum orvanadium oxides. More preferably the molybdenum and//or vanadium oxidesare dispersed on a layer or layers of a reducible oxide or oxides thatis further supported on alumina, titania, silica or zirconia (ifzirconia is not used as the above-mentioned layer).

[0017] Catalysts of this type that comprise molybdenum or vanadiumoxides supported on alumina or zirconia are described in several priorpublications, for catalyzing the oxidative dehydrogenation of propane topropene. These include Chen, et al., in “Studies in Surface Science andCatalysis”, Vol. 136, pp. 507-512, J. J. Spivey, E. Iglesia and T. M.Fleisch, Ed. (Elsevier Science, B. V., 2001); Chen et al., J. Catalysis189, 421 (2000), Khodakov et al., J. Catalysis 177, 343 (1998), Chen etal., J. Catalysis 198, 232 (2001) and Chen et al., J. Phys. Chem. B2011,105, 646 (2001). These publications are hereby incorporated herein byreference. However, these publications do not disclose catalystscontaining stannic oxide, titania, silica, or other supports, and do notdiscuss the usefulness or potential usefulness of the disclosedcatalysts for reactions such as the production of formaldehyde fromdimethyl ether.

[0018] In the catalysts of this invention, the molybdenum and/orvanadium oxide is distributed on the surface of the support material inwhat is known as “small domain” distribution. The surface density of theoxide catalyst on the support (measured in units of Mo or V metal atomsper nm²) is chosen so as to be greater than the surface density of therespective isolated monomeric oxide or oxides, but the catalyst overallis characterized by a substantial absence of bulk crystalline molybdenumand/or vanadium oxides (corresponding to the oxide or oxides used inpreparing the catalyst). By “bulk crystalline oxides” is meant oxide(s)having a clear X-ray diffraction pattern. The crystallinity can bedetermined by X-ray diffraction based on the peak intensity ratiobetween one of the peaks of the supported metal oxide and one of thepeaks of the support employed after calibration with a mixture of knownamounts of the metal oxide and the support. By “substantial absence” ismeant that the supported catalyst contains less than about 5% of bulkcrystalline molybdenum and/or vanadium oxide(s).

[0019] Surface densities of the catalysts in this invention are given interms of nominal surface density. This value is calculated based on theelemental analysis of the molybdenum and/or vanadium oxide and on thesurface area of the support, i.e., by dividing the number of metal atomsof the catalytic metal (Mo or V) in a given mass of sample by thesurface area of the support (calculated from N₂ absorption at its normalboiling point using the Brunauer-Emmett-Teller, or BET, equation). Wherethe metal oxide does not appreciably interact with the support to form acomplex (as described below), this calculated surface density fairlyclosely conforms to the actual surface density of the metal atoms on thesurface of the support. However, where an appreciable amount of acomplex is formed between the metal oxide and the support, the (nominal)surface density represents what that value would be were a complex notformed.

[0020] The surface density of the catalyst affects the catalystefficiency. At one extreme, catalysts of this type with relativelyisolated oxide species, for example monomolybdate or monovanadatespecies, have relatively few active sites on the support surface. Thesecatalysts tend to retain their oxygen and thus provide rather lowreaction rates for the oxidation of dimethyl ether to formaldehyde. Atthe other extreme, catalysts having bulk crystals may possibly providereasonable reaction rates per unit surface area. However, they also lackefficiency in the utilization of the oxide catalyst because asubstantial amount of the oxide is located within the crystals and isthus not available for catalyzing the reaction. Bulk MoO₃ crystals alsotend to be nonselective in their functioning, and can cause overreactionto produce carbon oxides rather than the desired products formaldehydeand methyl formate.

[0021] It has been found that the most preferred catalysts for thisreaction tend to have a surface density of approximately a monolayer ofcatalyst on the support. The monolayer surface density depends primarilyon the oxide chosen. For molybdenum oxide, the monolayer surface densityis ˜5.0 Mo atoms per square nanometer of support (Xie et al, Adv.Catal., 37, 1 (1990)). For vanadium oxide, this value is approximately7.5 V atoms per square nanometer (Centi, Appl. Catal. A, 147, 267(1996)). The term “monolayer” as used herein is meant to refer to theseapproximate surface densities. If the catalyst is uniformly dispersed onthe support, satisfactory results are obtainable with a preferredsurface density of from approximately 50-300% of the monolayer capacityvalues for alumina supports, and approximately 50-400% of these valuesfor zirconia supports. Overall, a preferred range of surface densitiesis from about 50 to about 300% of the monolayer capacity, for bothmolybdenum and vanadium oxides, for all supports usable in thisinvention.

[0022] The molybdenum or vanadium oxide may be present as the oxide perse, represented by the general formulas MoO_(x) and VO_(y), where x andy represent general values for oxygen in such molecules. For MoO_(x),the oxide generally comprises about three oxygen atoms per molybdenumatom; i.e., the general form of the oxide may be represented as MoO₃, ormolybdenum trioxide. For VO_(y), the oxide generally comprises aboutfive oxygen atoms per two vanadium atoms, represented by the generalformula V₂O₅, or vanadium pentoxide. However, in a given case the oxidemay have an oxygen-to-metal atomic ratio that is not necessarily exactly3:1 for molybdenum oxides or 5:2 for vanadium oxides. Likewise, oxidesused as a component of the support may be represented by more generalformulas such as SnO_(x), FeO_(x) and CeO_(x) where the oxides generallycomprise about 2, 1.5 and 2 oxygen atoms per metallic atom,respectively. However, in a given case, such oxide may have anoxygen-to-metal atomic ratio that is not exactly these values.

[0023] In addition, the molybdenum or vanadium oxides may form one ormore complexes or compounds with the support. These complexes usuallyalso will be an oxide such as polymolybdates and/or polyvanadates. Suchmolybdenum complexes may have general formulas such as ZrMo₂O₈. Vanadiumcomplexes would generally be represented by the formulaM_(2x)V_(2y)O_((nx+5y)) where M is the cationic ion of the support and nis the oxidation state of M, e.g., ZrV₂O₇. In any case, such complexesof molybdenum and vanadium oxides with the support are considered to bewithin the definition of the oxide catalysts to which this inventionpertains.

[0024] For instance, with molybdenum oxide supported on zirconia, asseen in examples below, where the Mo surface density is below 6.4 Mo/nm²the ZrO₂ surface is covered predominantly by two-dimensionalpolymolybdates (irrespective of the temperature of preparation), and theMoO_(x) domain size increases with increasing the Mo surface density. AtMo surface densities above 6.4 Mo/nm², increase in the Mo surfacedensity leads to the preferential formation of MoO₃ or ZrMo₂O₈crystallites on the ZrO₂ surface after treatment in air at 723 and 773 Kor at 873 K, respectively. This makes a fraction of the Mo activecenters inaccessible to dimethyl ether reactions and thus, as describedbelow, leads to a monotonic decrease in the primary dimethyl etherreaction rates with increasing Mo surface density (>6.4 Mo/nm²).

[0025] For such samples where the surface density was greater than 6.4Mo/nm², the areal dimethyl ether reaction rates (per surface area) andprimary selectivities approached constant values as the Mo surfacedensity increased. This indicates that the MoO₃ or ZrMo₂O₈ domains atthe ZrO₂ surface do not change in their local structure or surfaceproperties, while their domain size grows with increasing the Mo surfacedensity. The surface density of 6.4 Mo/nm² exceeds the theoreticalpolymolybdate monolayer, which is about 5.0 Mo/nm² Nevertheless, thecatalyst sample with a surface density of 6.4 Mo/nm² exhibited thehighest dimethyl ether reaction rates among the zirconia-supportedmolybdenum catalyst samples. This appears to be a compromise betweenreactivity and accessibility of the MoO_(x) sites. The samples having aZrMo₂O₈ structure possess a higher reactivity compared to the sampleshaving polymolybdates and MoO₃ crystallines at a given Mo surfacedensity, which is believed to be a result of the higher reducibility ofthe ZrMo₂O₈ species. The reducibility of the MoO_(x) domains(characterized by a H₂ temperature-programmed reduction method) wasfound also to be dependent on the domain size and structures of theMoO_(x) species. The larger MoO_(x) domains undergo faster reductioncompared to the smaller ones, and ZrMo₂O₈ domains are more reduciblethan two-dimensional polymolybdate and MoO₃ domains at a given Mosurface density, reflecting the difference in the ability of thesespecies to delocalize charge.

[0026] The support may be selected among commonly used supports for suchoxide catalysts, including mixtures of such supports, provided it allowsor favors the formation of a monolayer of molybdenum and/or vanadiumoxide on the surface of the support and otherwise is suitable for use inthe production of formaldehyde from dimethyl ether. Some properties maymake certain supports unsuitable for use in the process of thisinvention. For instance, supports that will react with the molybdenumand/or vanadium oxide to form any significant amounts of unreduciblemixed bulk oxides, i.e. oxides that would undergo substantial formationof oxygen vacancies at temperatures below about 300-400° C., would ingeneral not be suitable for use in this process. One commonly usedcatalyst support, magnesium oxide, for example, was tested forsuitability in this process and was found unsuitable. Supports thatcould cause undesired combustion of products to form carbon oxides underthe operating conditions of this process, or that contain acid sitesthat could cause formation of excessive amounts of methanol under theconditions of this process also would not be suitable for use in thisinvention.

[0027] The catalyst preferably contains molybdenum or vanadium oxide,but may contain a combination of the two. When both oxides are presentin the catalyst, one may be present as a layer of oxide on the support,preferably close to a monolayer, and the other present as a layer on topof the first oxide layer. Catalysts of this invention thus may comprisea layer, preferably approximately a monolayer, of one of molybdenum orvanadium oxide on a layer, preferably approximately a monolayer, of theother, on a support such as alumina or zirconia. The support mayoptionally further comprise a reducible metal oxide as described below.

[0028] Preferred supports include alumina, zirconia, titania, silica,and reducible metal oxides such as stannic oxide, ferric oxide, cericoxide, and mixtures or other combinations of two or more of theseoxides. Particularly preferred are alumina, zirconia and stannic oxide,arid mixtures or other combinations of two or all three of them. Mostpreferred is a catalyst comprising alumina, titania, zirconia or silicamodified by the incorporation of a layer or layers of a reducible oxidesuch as zirconia, stannic oxide, ferric oxide or ceric oxide depositedthereon. The supports that are suitable for use in this process may beused in any of their available forms, including forms that as of thepresent time might not yet have been developed, or may have beendeveloped but have not yet been commercialized. Both high and lowsurface area supports may be used, including materials known by theacronym MCM (standing for Mobil Compositions of Matter), e.g., MCM-41.These are recently developed mesoporous materials (often comprisingsilica) and are described in Kresge, et al., (Nature, 359, 710 (1992))and by Corma (Chem. Rev., 97, 2373 (1997)). High surface area supportsof various physical types are preferred for use in the invention fromthe point of view of efficiency in that they may produce greater amountsof product per unit mass of overall catalyst.

[0029] Reducible metal oxides suitable for inclusion in the catalysts ofthis invention are those in which at least a fraction of the metalcations undergo a one- or two-electron reduction during contact with areactant such as hydrogen, dimethyl ether, methanol, alkanes or alkenesat typical temperatures of catalytic oxidation reactions, whether or notsuch metal oxides function as catalyst for the reaction in question. Thefraction of the reducible metal oxide that undergoes such reduction neednot be large, as the effect of the reducible metal oxide is continuous.Such reducible metal oxides include reducible oxides of tin, iron,cerium, manganese, cobalt, nickel, chromium, rhenium, titanium, silverand copper, and mixtures thereof. Of these, oxides of tin (e.g., stannicoxide), iron (e.g., ferric oxide) and cerium (e.g., ceric oxide) arepreferred, with stannic oxide being most preferred for such catalysts ofthis invention.

[0030] Novel catalysts of this invention include those in which thesupport comprises a layer of a reducible metal oxide disposed on aparticulate alumina and/or zirconia (except where zirconia is used asthe above-mentioned layer), or a layer of zirconia disposed on aparticulate alumina, particularly those in which the layer of orzirconia has a surface density close to that of a monolayer of thatsubstance. Exemplary catalysts may comprise molybdenum and/or vanadiumoxide on a near-monolayer of stannic oxide disposed on a particulate(preferably high surface area) alumina. Novel catalysts of thisinvention also include those in which the reducible metal oxide oroxides is incorporated into the support.

[0031] Without intending to be bound by an explanation, it is believerthat the reducible metal oxides aid in catalyst performance bydecreasing the temperature required for the reduction of some of themolybdenum and/or vanadium atoms from their highest oxidation state.

[0032] The novel catalysts of this invention that contain reduciblemetal oxides also are suitable as catalysts for other reactions andprocesses, including but not limited to oxidation of methanol to produceformaldehyde, oxidative dehydrogenation of alkanes, and oxidation ofalkenes.

[0033] The catalysts of the invention are prepared by typical means, forinstance by impregnation, particularly incipient wetness impregnation,of the support with an aqueous solution containing molybdenum and/orvanadium, e.g. using a salt such as an ammonium molybdate or vanadate,for instance, ammonium di- or heptamolybdate or ammonium metavanadate.The preparation is carried out so as to disperse the molybdenum and/orvanadium oxide over the surface of the support and the amounts arechosen so as to achieve a desired surface density. Where the catalystalso comprises a reducible metal oxide, for instance as a layer on aparticulate support, the reducible metal oxide may be first deposited onthe particulate support, for instance by impregnation such as incipientwetness impregnation. Then the molybdenum and/or vanadium oxide isdeposited on the support in a second step, e.g. a second impregnation.Preparation of such catalysts by incipient wetness impregnation isdescribed in the Chen et al. and Khodakov et al. publications mentionedabove.

[0034] Catalysts of this invention may alternatively be prepared byother means such as chemical vapor deposition of layers, precipitation,sol-gel methods and the like. Reducible metal oxides may be incorporatedinto the catalysts either before or after the incorporation of themolybdenum and/or vanadium oxides.

[0035] The primary products of the reaction are formaldehyde and methylformate. Production of methyl formate can be increased if desired, bydecreasing the surface density of metal oxide or choosing a specificsupport such as stannic oxide and/or zirconia, or it may be decreased(which is generally preferred since formaldehyde is typically thepreferred product) by providing a catalyst having a surface densityclose to the value for a monolayer of catalyst, which, as will be shownbelow, generally has the highest selectivity to formaldehyde of thecatalysts of the invention. However, production of methyl formate is tobe expected in such a process, and is not especially detrimental asmethyl formate has uses of its own as a chemical intermediate and canreadily be separated from the reaction products and forwarded to otherprocess units for such uses.

[0036] Methanol is also produced in processes of this type, but itdehydrates relatively readily to re-form dimethyl ether. The methanolproduced can be recovered and recycled. Alternatively, methanol producedby this reaction may be forwarded to another unit, either for productionof further formaldehyde using a typical catalyst for that process, orfor other uses as a chemical intermediate. Methanol formation thereforecan be essentially disregarded in calculating selectivity of thedimethyl ether to formaldehyde.

[0037] The feed to the process may include, in addition to dimethylether, mixtures of dimethyl ether and methanol, provided that dimethylether is the major component of such mixtures. The oxidizing agent maybe air, oxygen-enriched air, or even pure oxygen (though this is likelyto be unnecessarily costly).

[0038] The process of this invention may be run in equipment ranging insize from microreactors (e.g. microchannel reactors) to full-sizedcommercial process equipment. A commercial installation will includetypical process expedients such as recycle streams, for efficient use ofreactants and reaction products, and may be integrated with processunits for production of dimethyl ether or for production of productsfrom formaldehyde.

[0039] As compared with data in patents mentioned above, the process ofthis invention exhibits both improved conversions of dimethyl ether aswell as improved selectivity to formaldehyde, and can achieve theseresults at lower temperatures. The process of this invention may beoperated in general at temperatures of from about 150 to about 400° C.,preferably from about 180 to about 350° C., most preferably from about150 to about 320° C. Operating pressures are about 0.1-100 atm,preferably about 1-20 atm. Residence time generally ranges from about 1to about 60 seconds.

EXAMPLES

[0040] The following are representative examples of this invention.These examples are provided by way of illustration only and not by wayof limitation. Those of skill will readily recognize a variety ofnoncritical parameters that could be changed or modified to yieldessentially similar results.

Example 1 Production of Formaldehyde from Dimethyl Ether usingMolybdenum Catalysts Supported on Alumina, Zirconia and Stannic Oxide

[0041] Supported MoO_(x) catalysts were prepared by incipient wetnessimpregnation of ZrO(OH)₂, SnO₂, or Al₂O₃, respectively, with aqueous(NH₄)₂Mo₂O₈ (99%, Aldrich) solutions (13-15). ZrO(OH)₂ was prepared byhydrolysis of aqueous zirconyl chloride solutions (>98%, Aldrich) usingNH₄OH (14.8 N), followed by drying in ambient air at 393 K overnight.SnO₂ was prepared by hydrolysis of tin (IV) chloride pentahydrate (98%,Alfa Aesar) with NH₄OH (14.8 N), followed by treatment in flowing dryair at 773 K for 3 h. A commercial source of γ-Al₂O₃ (Degussa A G) wasused without further treatment. All samples were dried afterimpregnation at 373 K in ambient air and then treated in flowing dry airat 773 K for 3 h. Bulk MoO₃ powders were prepared by decomposition of(NH₄)₂Mo₂O₈ (99%, Aldrich) in flowing dry air at 773 K for 3 h.

[0042] Dimethyl ether reactions were carried out in a fixed-bed quartzmicroreactor using catalysts (0.3 g) diluted with quartz powder (1 g) inorder to prevent local high temperatures caused by the exothermic natureof the reaction. The reactant mixture consisted of 80 kPa DME and 18 kPaO₂, and 2 kPa N₂ was used as an internal standard. Reactants andproducts were analyzed by on-line gas chromatography (Hewlett-Packard6890 GC) using flame ionization and thermal conductivity detectors andmethyl-silicone capillary and Porapak® Q packed columns.

[0043] Table 1 shows catalytic results obtained at 513 K on MoO_(x)domains supported on Al₂O₃, ZrO₂ and SnO₂ with similar Mo surfacedensities (6.4-7.0 Mo-atoms/nm²), alone and compared with resultsreported in previous patents. Rates and selectivities (in all tables)were measured as a function of DME conversion, which was changed byvarying the reactant residence time. DME conversion rates andformaldehyde selectivities were extrapolated to zero reactant residencetime in order to obtain primary DME conversion rates and selectivities.DME conversion rates and selectivities are reported in two forms in theresults shown in Table 1. One approach considers CH₃OH as a DMEconversion product; the other approach reports rates and selectivitieson a CH₃OH-free basis, which seems appropriate in view of the reversiblenature of DME conversion to methanol and the pathways available for theultimate conversion of both CH₃OH and DME to HCHO.

[0044] Primary reaction rates (normalized by catalyst mass) were muchhigher on the three catalysts of the invention than on catalystspreviously reported in the patents, even at the lower temperatures usedhere. Rates were higher on SnO2 and ZrO₂ supports than on Al₂O₃, but theprimary formaldehyde selectivity (CH₃OH-free) was almost 100% onMoO_(x)/Al₂O₃. Pure supports showed very low DME conversion rates. AMoO₃ sample with relatively low surface area gave a low DME conversionrate (per gram), but its areal rate resembled those on MoO_(x)/Al₂O₃ andwas 2-6 times lower than on MoO_(x) supported on ZrO₂ and SnO₂. Thus,DME conversion appears to require small MoO_(x) domains with muchgreater accessibility to reactants than those available in bulk MoO₃samples. Formaldehyde was not detected on MoO₃ at 513 K, because of thelow DME conversions attained. At higher temperatures (593 K), theprimary HCHO selectivity was 52.9% (on a CH₃OH-free basis) on bulk MoO₃.MoO_(x)/Al₂O₃ was the most selective catalyst for DME conversion toHCHO. Its primary HCHO selectivity was 79.1% (98.1%, CH₃OH-free basis)and CO and CO₂ (CO_(x)) were not detected as primary products.

[0045] Included in these tests was a catalyst similarly preparedcomprising molybdenum oxide supported on magnesium oxide. MgO wasprepared by contacting MgO (>98%, Aldrich) with deionized water at355-365 K for 4 h, followed by treatment in flowing dry air at 773 K for8 h. As seen from Table 1, however, MoO_(x) domains supported on MgO didnot give detectable DME conversion rates, apparently as a result of theformation of mixed metal oxides, which are unable to undergoreduction-oxidation cycles required for catalytic DME conversionturnovers at these temperatures. This support thus appears unsuitablefor use with the catalysts in this process. TABLE 1 DME oxidation ratesand selectivities on supported MoO_(x) catalysts at 513 K (80.0 kPa, 18kPa O₂ and 2 kPa N₂), on bulk MoO₃, on pure supports, and on previouslyreported catalysts. DIMETHYL ETHER DME DME Mo Conversion ConversionConversion Catalyst Surface surface Temper- Rate^(d) Rate^(d) Rate^(d)(MoO₃ area density ature (mmol/ (mol/g-atom (10⁻⁵ Selectivity (%)^(d) wt%) (m²/g) (Mo/nm²) (K) g_(-cat)-h) Mo-h) mol/m²-h) CH₃OH HCHO MF^(a)DMM^(b) CO_(x) ^(c) Reference MoO₃/ZrO₂ 136.3 6.4 513 17.6 12.2 12.922.6 53.4 16.8 0.3 6.9 This study (20.7%) (13.6) (9.4)  (69.0) (21.7)(0.4)  (8.9)  MoO₃/SnO₂ 46.5 6.5 513 18.4 36.5 39.6 13.1 67.3 9.6 0 9.8This study (7.2%) (16.0) (31.7) (77.6) (11.1) (0)   (11.3) MoO₃/Al₂O₃90.0 7.0 513 5.8 5.7 6.6 19.7 79.9 1.6 0 0 This study (15.0%) (4.7) (4.6)  (98.1) (1.9)  (0)   (0)   MoO₃/MgO 171.2 5.8 513 not detected notdetected — — — — — — This study (24.0%) ZrO₂ 106.4 — 513 not detected —— — — — — — This study SnO₂ 48.8 — 513 0.6 — 1.2 29.7 0 0 0 70.3 Thisstudy Al₂O₃ 110 — 513 0.2 — 0.2 82.9 0 0 0 17.1 This study MoO₃ 3.3 —513 0.2 — 6.1 6.0 0 0 0 94.0 This study ^(e)Ag — — 887 10.5 — — — 45.8 00 30.7 10 ^(f)Bi—Mo— — — 773 3.1 — — 0 46.0 0 0 54.0 7 FeO_(x)^(g)Bi—Mo— — — 773 2.9 — — 0 43.0 0 0 55.0 8 CuO_(x) ^(h)Mn nodules ˜230— 623 1.7 — 0.7 — 49.0 — — — 9

Example 2 Additional Tests with Molybdenum Oxide Catalysts Supported onAlumina

[0046] A parallel study showed that the catalytic properties of theseMoO_(x) domains depend sensitively on their size and local structure,which were varied by changing the Mo surface density on Al₂O₃ (1.6-11.3Mo/nm²) and ZrO₂ (2.2-30.6 Mo/nm²). Primary DME reaction rates increasedfrom 2.3 to 5.7 mol/g-atom Mo-h as the Mo surface density increased from1.6 to 7.0 Mo/nm² on Al₂O₃ (Table 2). These rates increased from 0.6 to12.2 mol/g-atom Mo-h as the Mo surface density increased from 2.2 to 6.4Mo/nm² on ZrO₂. On both ZrO₂ and Al₂O₃, DME conversion rates per Moreached maximum values at surface densities of 6-7 Mo/nm². Results arereported in Table 2.

[0047] X-Ray diffraction and Raman, UV-visible, and X-ray absorptionspectroscopies did not detect MoO₃ crystallites in samples with surfacedensities below 7 Mo/nm². In this Mo surface density range, most, if notall, MoO_(x) species are accessible at surfaces and the DME conversionrates per Mo atom are equivalent to rates per exposed MoO_(x) moiety(i.e. turnover rates). Therefore, the higher reaction rates attainedwith increasing surface density reflect a higher reactivity of exposedMoO_(x) as the size and dimensionality of MoO_(x) domains increases withincreasing Mo surface density. The larger domains formed at higherMoO_(x) surface densities (detected by measurements of their absorptionedge energy in the UV-visible spectra) appear to undergo the redoxcycles required for DME conversion to HCHO with greater ease thanisolated monomolybdate species or smaller two-dimensional polymolybdatedomains.

[0048] This interpretation is consistent with the observed decrease inthe temperature required for H₂ reduction of Mo⁶⁺ to Mo⁴⁺ in thesesamples. Ultimately, DME conversion rates (per Mo) decreased at evenhigher Mo surface densities (>10 Mo/nm²), because the incipientformation of three-dimensional MoO₃ clusters leads to increasinglyinaccessible MoO_(x) species.

[0049] Primary formaldehyde selectivities also increased monotonicallywith increasing Mo surface density; they reached 79.1% (98.1% on aCH₃OH-free basis) on MoO_(x)/Al₂O₃ at 11.3 Mo/nm². Methanolselectivities decreased as the Al₂O₃ support was covered with MoO_(x)species, and the primary formaldehyde selectivity concurrently increasedwith increasing Mo surface density. Methyl formate and CO_(x) primaryselectivities were very low on all Al₂O₃-supported MoO_(x) samples. OnAl₂O₃-supported samples with surface densities of 1.6 to 11.3 Mo/nm²,the CH₃OH-free primary HCHO selectivity was 95.2-98.1% (Table 2). TABLE2 Effects of surface density of MoO_(x)/Al₂O₃ catalysts on primary DMEreaction rates and primary products at 513 K (80.0 kPa, 18 kPa O₂ and 2kPa N₂). MoO₃ loading Mo surface density Rate^(d) Rate^(d) Selectivity(%)^(d) (MoO₃ wt %) (Mo/nm²) (mol/g-atom Mo-h) (10⁻⁵ mol/m²-h) CH₃OHHCHO MF^(a) DMM^(b) CO_(x) ^(c)  4.0% 1.6 2.3 0.6 34.1 62.8 2.5 0   0.8 (1.5)^(d)  (0.4)^(d) (95.2) (3.7) (0)   (1.2)  8.0% 3.4 3.9 2.2 27.070.5 2.4 0   0.2 (2.8) (1.6) (96.5) (3.2) (0)   (0.3) 10.0% 4.5 5.2 4.022.1 76.3 1.5 0   0.1 (4.1) (3.1) (97.9) (1.9) (0)   (0.1) 15.0% 7.0 5.76.6 19.5 79.1 1.6 0   0   (4.6) (5.3) (98.1) (1.9) (0)   (0)   20.0%11.3 3.8 7.1 19.4 79.0 1.6 0.1 0   (3.1) (5.7) (98.0) (1.9) (0.1) (0)  

Example 3 Effect of Temperature on Reaction Rates and Primary Products

[0050] Table 3 shows results of a study comparing reaction rates andselectivity at temperatures of 473-533 K (200-250° C.). The catalystused contained 15 wt. % MoO₃ on alumina with a surface density of 7.0Mo/nm². As the temperature was increased from 473 to 533 K, the reactionrate increased significantly, and selectivity to formaldehyde (asopposed to methyl formate) also increased significantly. TABLE 3 Effectsof temperature on primary DME reaction rates and primary products onMoO_(x)/Al₂O₃ catalyst (15 wt. % MoO₃; 7.0 Mo/nm²) (80.0 kPa, 18 kPa O₂and 2 kPa N₂). Rate^(d) (mol/ Selectivity (%)^(d) Temperature g-atomMo-h) CH₃OH HCHO MF^(a) DMM^(b) CO_(x) ^(c) 473 1.3 24.2 69.2 4.2 0.22.1 (1.0)  (91.2) (5.5) (0.3) (2.8) 493 2.9 23.8 73.1 2.0 0 1.2 (2.2) (95.9) (2.6) (0)   (1.6) 513 5.7 19.5 79.1 1.6 0 0 (4.6)  (98.1) (1.9)(0)   (0)   533 12.9 17.3 81.0 1.8 0 0 (10.7) (97.9) (2.1) (0)   (0)  

Example 4 Further Experiments with Molybdenum Oxide Catalysts Supportedon Zirconia

[0051] A series of molybdenum oxide catalysts supported on zirconia,having a range of surface densities and prepared by calcining at twodifferent temperatures was used to catalyze the production offormaldehyde from dimethyl ether.

[0052] The catalysts were prepared by incipient wetness impregnation ofprecipitated zirconium oxyhydroxide (ZrO(OH)₂) with an aqueous solutionof ammonium dimolybdate [(NH₄)₂Mo₂O₈] (99%, Aldrich). Zirconiumoxyhydroxide (ZrO(OH)₂) was prepared by precipitation of a zirconylchloride solution (98%, Aldrich) at a constant pH of 10 by controllingthe rate of addition of ammonium hydroxide solution (29.8%, FisherSci.). After precipitation, the solids were washed with mildly basicammonium hydroxide solution (pH˜6) until the effluent showed no chlorideions tested by a silver nitrate solution. The resulting solids weredried in air overnight at 393 K (120° C.). The dried solids wereimpregnated with an aqueous solution of ammonium dimolybdate at roomtemperature. The Mo⁶⁺ concentrations in the solution were varied inorder to get desired Mo content in the final catalysts. Afterimpregnation, samples were dried in air at 393 K and treated in dry airat 723, 773 or 873 K (450, 500 and 600° C.) for 3 h.

[0053] The catalysts were systematically characterized by means ofpowder X-ray diffraction (XRD), diffuse reflectance UV-visible, Raman,and X-ray absorption (XANES/XAFS) spectroscopies. Surface areas weremeasured by N2 physisorption using standard multipoint BET method. Mosurface density is expressed as the number of Mo atoms per squarenanometer BET surface area (Mo/nm²). The catalysts so prepared arelisted in Table 4. TABLE 4 Surface areas and Mo surface density forMoO_(x)/ZrO₂ catalysts treated at 723, 773 and 873 K. 723 K 773 K 873 KMoO₃ Loading Surface area Mo surface density Surface area Mo surfacedensity Surface area Mo surface Sample (wt. %) (m²/g) (Mo/nm²) (m²/g)(Mo/nm²) (m²/g) density (Mo/nm²)  1MoO_(x)/ZrO₂  1.0% 118.8 0.3 105.60.4 85.6 0.5  6MoO_(x)/ZrO₂  5.7% 130.0 1.8 110.3 2.2 97.1 2.511MoO_(x)/ZrO₂ 11.0% 145.9 3.2 132.6 3.5 103.4 4.5 21MoO_(x)/ZrO₂ 20.7%153.7 5.6 136.3 6.4 102.7 8.4 29MoO_(x)/ZrO₂ 29.3% 114.0 10.7 96.9 12.664.6 20.0 37MoO_(x)/ZrO₂ 37.0% 99.6 15.5 73.9 20.9 49.3 31.444MoO_(x)/ZrO₂   44% 83.5 22.0 60.2 30.6 36.7 50.1

[0054] Tables 5 and 6 show the results using catalysts so prepared andcalcined at 773 K and 873 K, respectively. Both catalysts demonstratedvery good dimethyl ether conversion rates and selectivity, with betterperformance being exhibited in general by the catalyst that had beencalcined at 873 K. TABLE 5 Effects of surface density of MoO_(x)/ZrO₂catalysts (calcined at 773 K) on primary DME reaction rates and primaryproducts at 513 K (80.0 kPa, 18 kPa O₂ and 2 kPa N₂). MoO₃ loading Mosurface density Rate^(d) Rate^(d) Selectivity (%)^(d) (MoO₃ wt. %)(Mo/nm²) (mol/g-atom Mo-h) (10⁻⁵ mol/m²-h) CH₃OH HCHO MF^(a) DMM^(b)CO_(x) ^(c) 11.0% 3.5 8.5 4.9 24.0 32.3 30.0 0 13.7 (6.5) (3.7)  (42.5)(39.5) (0)   (18.1) 20.7% 6.4 12.2 12.9 22.6 53.4 16.8 0.3 6.9 (9.4)(10.0) (68.9) (21.7) (0.4) (8.9)  29.3% 12.6 4.1 8.6 22.3 62.9 8.4 0.26.4 (3.2) (6.7)  (80.8) (10.7) (0.2) (8.2)  37.0% 20.9 3.2 11.1 23.066.9 7.6 0.1 2.4 (2.5) (8.6)  (86.9) (9.8)  (0.1) (3.1)  44.0% 30.6 2.010.4 22.4 68.6 5.0 0 4.0 (1.6) (8.1)  (88.4) (6.5)  (0)   (5.1) 

[0055] TABLE 6 Effects of surface density of MoO_(x)/ZrO₂ catalysts(calcined at 873 K) on primary DME reaction rates and primary productsat 513 K (80.0 kPa, 18 kPa O₂ and 2 kPa N₂). MoO₃ loading Mo surfacedensity Rate^(d) Rate^(d) Selectivity (%)^(d) (MoO₃ wt. %) (Mo/nm²)(mol/g-atom Mo-h) (10⁻⁵ mol/m²-h) CH₃OH HCHO MF^(a) DMM^(b) CO_(x) ^(c)11.0% 4.5 23.0 17.0 24.9 46.0 23.5 0.4 5.2 (17.3) (13.0) (61.2) (31.3)(0.5) (7.0) 20.7% 8.4 46.5 51.7 23.7 60.1 11.5 0.8 4.0 (35.5) (39.5)(78.6) (15.1) (1.1) (5.2) 29.3% 20.0 19.0 60.0 24.1 59.6 12.2 0.9 3.2(14.4) (45.5) (78.5) (16.1) (1.2) (4.2) 37.0% 31.4 11.7 61.1 24.9 58.312.6 1.1 3.0 (8.8)  (45.9) (77.7) (16.8) (1.5) (4.0) 44.0% 50.1  7.461.3 27.8 60.1 10.4 0.9 1.3 (5.3)  (44.3) (82.7) (14.4) (1.2) (3.8)

[0056] In other work, the effect of partial pressures on the reactionwas investigated on the 44 MoO_(x)/ZrO₂ catalyst (50.1 Mo/nm²). Thereaction rates nearly increased linearly as the DME partial pressureincreased from 10 to 40 kPa, and then approached the constant valuesabove 60 kPa. The primary selectivities to methyl formate anddimethoxymethane were almost independent of the DME partial pressure.The primary selectivity to CO_(x) decreased from 6.0% to the constantvalue of 1.5% while the selectivity to HCHO increased from 78.3 to 82.7%with increasing the DME pressure from 10 to 40 kPa.

Example 5 Preparation and Use of Supported Vanadium Pentoxide Catalysts

[0057] The catalysts were prepared by incipient wetness impregnation ofγ-Al₂O₃ (Degussa A G) with an aqueous solution of ammonium metavanadate[NH₄NO₃] (99%, Aldrich) and oxalic acid (Mallinckrodt, analytical grade;NH₄/NO₃/oxalic acid=0.5 M) (the addition of oxalic acid improves thedissolution of NH₄NO₃ in water). The V⁵⁺ concentrations in the solutionwere varied in order to get desired V content in the final catalysts.After impregnation, samples were dried in air at 393 K and treated indry air at 773 K (500° C.) for 3 h.

[0058] On VO_(x)/Al₂O₃ (8.0 V/nm ²) the CH₃OH-free primary HCHOselectivity was 99.6% at 513 K and the primary DME reaction rate was 6.8mol/g-atom V-h. Results are shown in Table 7. TABLE 7 Primary DMEreaction rates and primary products on VO_(x) dispersed on differentsupports at 240° C. (80.0 kPa, 18 kPa O₂ and 2 kPa N₂). Catalyst Vsurface density Rate^(a) Rate^(a) Selectivity (%) (V₂O₅ wt. %) (V/nm²)(mol/g-atom V-h) (10⁻⁵ mol/m²-h) CH₃OH HCHO MF^(b) DMM^(c) CO_(x) ^(d)VO_(x)/Al₂O₃ 8.0 8.0 6.7 14.72 84.96 0.26 0 0 (10.0%) (6.8)  (5.6)(99.61) (0.3) VO_(x)/ZrO₂ 6.2 13.1 8.5 5.2 37.9 9.2 0 47.7 (15%) (12.4)(8.1) (40.0)  (9.7) (50.3) VO_(x)/MgO 5.5 1.8 1.0 11.0 0 0 0 89.0(20.0%)

Example 6 Use of Mixed Oxide Catalysts

[0059] Similarly, experiments were conducted using a catalyst containingmolybdenum oxide on a support of alumina modified by stannic oxide,cerium oxide and ferric oxide.

[0060] SnO_(x) and ZrO_(x)-modified Al₂O₃ supports (SnO_(x)/Al₂O₃ andZrO_(x)/Al₂O₃) were prepared by incipient wetness impregnation of Al₂O₃(Degussa A G, ˜100 m²/g or 180 m²/g) with isobutanol solutions ofSn(i-C₄H₉O)₄ and Zr(i-C₄H₉O)₄ (Aldrich, 99.8%), respectively at 289 Kunder dry N₂ atmosphere for 5 h, followed by drying at 393 K overnightand then treating in flowing dry air (Airgas, zero grade) at 673 K for 3h. CeO_(x) and FeO_(x)-modified Al₂O₃ supports (CeO_(x)/Al₂O₃ andFeO_(x)/Al₂O₃) were prepared by incipient wetness impregnation of Al₂O₃(Degussa A G, 100 m²/g) with aqueous solutions of Ce(NO₃)₄ (Aldrich,99.99%) and Fe(NO₃)₃ (Aldrich, 99.9%), respectively at 289 K for 5 h,followed by drying at 393 K overnight and then treating in flowing dryair (Airgas, zero grade) at 673 K for 3 h. SnO₂ was prepared byhydrolysis of tin (IV) chloride pentahydrate (98%, Alfa Aesar) at a pHof ˜7 using NH₄OH (14.8 N, Fisher Scientific). The precipitates werewashed with deionized water until the effluent was free of chlorideions. The resulting solids were treated in flowing dry air (Airgas, zerograde) at 773 K for 3 h. Supported MoO_(x) catalysts were prepared byincipient wetness impregnation method using aqueous (NH₄)₆Mo₇O₂₄(Aldrich, 99%) solutions. Supported VO_(x) catalysts were also preparedby incipient wetness impregnation method using an aqueous solution ofammonium metavanadate [NH₄NO₃] (Aldrich, 99%) and oxalic acid(Mallinckrodt, analytical grade; NH₄NO₃/oxalic acid=0.5 M). All sampleswere dried at 393 K in ambient air after impregnation and then treatedin flowing dry air (Airgas, zero grade) at 773 K for 3 h. The Mo or Vsurface density for all supported samples is reported as Mo/nm² orV/nm², based on the Mo or V content and the BET surface area for eachsample.

[0061] The catalysts retained the good selectivity of the molybdenumoxide/alumina catalysts but had higher activity. Results are reported inTables 8 and 9. Table 10 shows results using vanadium oxide catalysts onalumina and on stannic oxide/alumina. As shown in Table 10, the DMEconversion rates using VO_(x)/SnO_(x)/Al₂O₃ (5.5 Sn/nm2) were about 2.4times than the rates on VO_(x)/Al₂O₃ at 513K. TABLE 8 Primary DMEconversion rates and product selectivities for MoO_(x) domains supportedon Al₂O₃ modified with SnO_(x) at different surface densities (1.5-11.2Sn/nm²) and also on unmodified Al₂O₃ and SnO₂ (˜7.0 Mo/nm²; 513 K; 80.0kPa DME, 18 kPa O₂ and 2 kPa N₂). Support (MoO₃ Sn surface density^(a)DME conversion rate ^(a)Primary selectivity (%) wt %) (Sn/nm²)(mol/g-atom Mo-h) HCHO ^(b)MF ^(c)DMM ^(d)CO_(x) Al₂O₃ 0 4.6 98.1 1.9 00 (15.0%) MoO_(x)/SnO₂ — 36.4 70.4 12.5 0 17.2 (5.9%) SnO_(x)/Al₂O₃ 1.55.4 97.2 2.8 0 0 (15.0%:) SnO_(x)/Al₂O₃ 2.8 7.1 98.0 2.0 0 0 (15.0%)SnO_(x)/Al₂O₃ 5.5 12.2 97.7 2.3 0 0 (15.0%) SnO_(x)/Al₂O₃ 11.2 12.9 97.62.4 0 0 (13.6%)

[0062] TABLE 9 Primary DME conversion rates and product selectivitiesfor MoO_(x) domains supported on unmodified Al₂O₃, and on Al₂O₃ modifiedwith near-single monolayer SnO_(x), ZrO_(x), CeO_(x), and FeO_(x) (513K; 80.0 kPa DME, 18 kPa O₂ and 2 kPa N₂). Catalyst Mo surface density^(a)DME conversion rate ^(a)Primary selectivity (%) (MoO₃ wt %) (Mo/nm²)(mol/g-atom Mo-h) HCHO ^(b)MF ^(c)DMM ^(d)CO_(x) MoO_(x)/Al₂O₃ 7.0 4.698.1 1.9 0 0 (15.0%) MoO_(x)/SnO_(x)/Al₂O₃ 7.1 12.2 97.7 2.3 0 0 (15.0%)MoO_(x)/ZrO_(x)/Al₂O₃ 6.8 8.7 98.6 1.4 0 0 (15.0%) MoO_(x)/CeO_(x)/Al₂O₃6.6 6.8 98.8 1.2 0 0 (13.4%) MoO_(x)/FeO_(x)/Al₂O₃ 6.7 6.2 99.7 0.3 0 0(14.1%)

[0063] TABLE 10 Primary DME conversion rates and product selectivitieson VO_(x) domains at near-single monolayer V surface density supportedon unmodified and SnO_(x)-modified Al₂O₃ (5.5 Sn/nm²) (513 K; 80.0 kPaDME, 18 kPa O₂ and 2 kPa N₂). Catalyst Mo surface density ^(a)DMEConversion Rate ^(a)Primary selectivity (%) (MoO₃ wt %) (Mo/nm²)(mol/g-atom Mo-h) HCHO ^(b)MF ^(c)DMM ^(d)CO_(x) VO_(x)/Al₂O₃ 8.0 6.899.6 0.4 0 (10.0%) VO_(x)/SnO_(x)/Al₂O₃ 7.9 16.3 97.7 2.3 0 (10.1%)

[0064] Tables 11 and 12 show the effects of BET surface area of theMoO_(x) and VO_(x) catalysts on the primary DME conversion rates andproduct selectivities at 513 K. The rates normalized by the mass of thecatalysts were proportional to their surface areas, while the ratesnormalized by Mo or V atoms and the HCHO selectivities were essentiallyindependent of the surface area of the catalysts.

[0065] For example, on MoO_(x)/Al₂O₃(B), the DME conversion rate pergram catalyst increased from 4.7 mmol/g-cat-h to 9.4 mmol/g-cat-h, i.e.,by a factor of two as the surface area increased from 90.0 m²/g forMoO_(x)/Al₂O₃(A) to 174.9 m²/g. The rate per Mo atom (4.7 vs. 5.1mmol/g-atom Mo-h) and the primary HCHO selectivity (98.1% vs. 96.0%)values remained essentially unchanged, reflecting no change in thecatalytic properties of the active MoO_(x) sites with changing thesurface area of the samples. TABLE 11 Surface area effects on primaryDME conversion rates and product selectivities for MoO_(x) domains atnear one monolayer surface density supported on unmodified andSnO_(x)-modified Al₂O₃ (˜5.5 Sn/nm²) (513 K; 80.0 kPa DME, 18 kPa O₂ and2 kPa N₂). BET surface Mo surface ^(a)DME conversion ^(a)DME conversion^(a)Primary HCHO selectivity area density rate rate (%) Support(m²/g-cat) (Mo/nm²) (mmol/g-cat-h) (mol/g-atom Mo-h) HCHO ^(b)MF ^(c)DMM^(d)CO_(x) Al₂O₃ (A) 90.0 7.0 4.7 4.6 98.1 1.9 0 0 (15.0%) Al₂O₃ (B)174.7 6.4 9.4 5.1 95.9 3.1 0 0 (26.8%) SnO₂/Al₂O₃ (A) 87.9 7.1 12.6 12.197.7 2.3 0 0 (15.0%) SnO₂/Al₂O₃ (B) 150.3 6.4 18.4 11.4 98.2 1.8 0 0(22.9%)

[0066] TABLE 12 Surface area effects on primary DME conversion rates andproduct selectivities for VO_(x) domains at near one monolayer surfacedensity supported on unmodified and SnO_(x)-modified Al₂O₃ (˜5.5 Sn/nm²)(513 K; 80.0 kPa DME, 18 kPa O₂ and 2 kPa N₂). BET surface V surface^(a)DME conversion ^(a)DME conversion ^(a)Primary HCHO selectivitySupport area density rate rate (%) (V₂O₅%) (m²/g-cat) (V/nm²)(mmol/g-cat-h) (mol/g-atom V-h) HCHO ^(b)MF ^(c)DMM ^(d)CO_(x) Al₂O₃ (A)83.0 8.0 6.7 6.8 99.6 0.4 0 0 (10.0%) Al₂O₃ (B) 195.9 7.8 13.5 5.9 97.12.9 0 0 (23.2%) SnO₂/Al₂O₃ (A) 84.3 7.9 16.2 16.3 97.7 2.3 0 0 (10.1%)SnO₂/Al₂O₃ (B) 149.2 7.5 25.3 15.3 98.0 2.0 0 0 (16.8%)

[0067] All publications and patent applications cited in thisspecification are herein incorporated by reference as if each individualpublication or patent application were specifically and individuallyindicated to be incorporated by reference.

[0068] Although the foregoing invention has been described in somedetail by way of illustration and example for purposes of clarity ofunderstanding, it will be readily apparent to those of ordinary skill inthe art in light of the teachings of this invention that certain changesand modifications may be made thereto without departing from the spiritor scope of the appended claims.

1-59. (canceled).
 60. A catalyst comprising molybdenum oxide, vanadiumoxide, or a mixture of molybdenum oxide and vanadium oxide supported ona support comprising one or more layers comprised of a reducible metaloxide or a mixture of reducible metal oxides, the reducible oxide layeror layers being disposed on a particulate alumina or zirconia support,in which the surface density of the molybdenum and/or vanadium oxide oroxides on the support is greater than that for the respective monomericisolated oxide or oxides, and the catalyst is characterized by asubstantial absence of bulk crystalline molybdenum and/or vanadiumoxides.
 61. A catalyst according to claim 60 in which the reduciblemetal oxides are selected from reducible oxides of tin, iron, cerium,manganese, cobalt, nickel, chromium, rhenium, titanium, silver andcopper, and mixtures thereof.
 62. A catalyst according to claim 60 inwhich the reducible metal oxide is selected from oxides of tin, iron,cerium, and mixtures thereof.
 63. A catalyst according to claim 60 inwhich the reducible metal oxide comprises stannic oxide.
 64. A catalystaccording to claim 60 in which the surface density of the molybdenumand/or vanadium oxide on the support is from about 50% of the surfacedensity of a monolayer of the oxide or oxides to about 300% of thesurface density of a monolayer of the oxide or oxides.
 65. A catalystaccording to claim 60 in which the surface density of the molybdenumand/or vanadium oxide or oxides on the support is approximately that ofa monolayer of the oxide or oxides at the surface of the support.
 66. Acatalyst according to claim 60 comprising molybdenum oxide supported ona layer of stannic oxide that is disposed on a particulate aluminasupport, and in which the surface density of the molybdenum-oxide isfrom about 1.5 to about 20 Mo/nm².
 67. A catalyst according to claim 60comprising molybdenum oxide supported on a layer of stannic oxide thatis disposed on a particulate alumina support, and in which the surfacedensity of the molybdenum oxide is approximately that of a monolayer ofthe oxide or oxides at the surface of the support.